History of Flight
On 9 May 2011, around 9:00 UTC, a Cessna U206G aircraft registered OO-A** operated by Aviation Sans Frontières Belgique was on a flight from Mbandaka to Basankusu airfield in the Democratic Republic of Congo. The flight carried four passengers and one pilot. After a normal takeoff and climb, the aircraft cruised at flight level 075 without incident. Approximately 30 nautical miles from Basankusu, a descent was initiated to 2000 feet, followed by an entry into right downwind for runway 24. At 1000 feet AGL, with all engine parameters normal (2400 RPM, 21 inHg, 100 kts, 10° flaps), the engine suddenly lost all power. The propeller stopped turning within 4 to 10 seconds. The pilot focused on flying, performed a steep descent, and landed successfully on runway 06, stopping at the end of the runway without damage.
Injuries and Damage
No injuries were reported among the five persons on board. The aircraft itself was not damaged. However, an on-site examination revealed a hole in the top of the right-hand crankcase above cylinders number 1 and 3. Subsequent disassembly showed the crankshaft fractured at cheek number 3, between the main and rod journal number 2.
Aircraft and Engine Information
The airframe was a Cessna U206G, serial number U20603690, built in 1977. Total airframe time was 5169:13 flight hours. The engine was a Teledyne Continental Motors IO-520-F (9), serial number 286023-R. It had accumulated 1425 flight hours since its last overhaul in February 2005, and 3110 hours since its factory rebuilding in April 1991. The crankshaft had a total time of approximately 3110 hours. No propeller strike had been recorded since the aircraft was registered as OO-A**. Maintenance was performed by a BCAA-approved Part M Subpart F organization. The last 200-hour inspection was conducted on 11 April 2011, 18 flight hours before the failure; the oil filter was cut and no anomalies were found.
Investigation Findings
The engine was sent to Belgium and disassembled on 24 November 2011. A thorough visual inspection confirmed the crankshaft fracture at cheek number 3. The investigation noted that an unapproved sealant had been applied to the parting surfaces of the crankcase halves during the last engine overhaul performed by Air Service International (EASA Part 145 facility DK.145.0007) in February 2005. This sealant was not approved by the engine manufacturer.
Probable Cause
The probable root cause of the crankshaft fracture is the installation, during the last engine overhaul, of an unapproved sealant on the parting surfaces of the crankcase halves.